Apparatuses and methods have been developed for testing devices having a display without opening the device or connecting any measuring equipment to the device. Such apparatuses may capture images of the display with an image sensor. Captured image information is then analysed to determine the quality of video playback. In some methods a certain area of the display is used to display a frame indicator. The frame indicator is for example a square which is shown as a white square by every other frame and as a black square by every other frame. Hence, the area can be imaged at intervals wherein the frame rate can be detected on the basis of the frequency of the varying black and white squares. However, it may happen that the display may not show each consecutive frame i.e. one or more frames may be dropped. This means that two consecutive frames which are shown by the display (and the dropped frame in between) may not be recognized as different frames because the square has the same colour in both of these frames. As a conclusion, the apparatus may interpret these two frames as one single frame, which causes erroneous test results. In some cases, the frame indicator may be a number, binary ID or even a rotating symbol. Problem with these larger indicators is that they consume the display space which should be reserved for the display content. They also require quite complex hardware for reading and analysing the frame information.
The document WO 2006/024698 discloses a method and apparatus for measuring the quality of a video transmission. To facilitate the measurement of the quality of a video transmission, a bar code is embedded in the top part of the frames of the transmitted video signal to enable identification of each frame. The audio signal may be synchronised with the different bar codes. The bar code may be formed of several blocks in the top of the image, and may be formed of superimposed binary bar codes, each binary bar code being in one colour of a colour system. This permits measurement of transmission quality, and notably of dropped frames, audio holes, transmission delay, audio and video synchronisation. The bar code can be added to a video signal to be transmitted e.g. by using a board comprising different light sources and capturing the lights by a camera, or they may be otherwise embedded in the video. A received video transmission can be examined by using an AV board, which uses the vertical and horizontal synchronisation signals to select the video lines where the bar code should be sampled. This kind of arrangement requires a separate board which should be connected to signal wiring before the display. Furthermore, this system is not able to test properties of the display because the measurement is performed before the video is provided to the display and the measurement is not based on information shown on the display
Therefore, there is a need to find an improved method and apparatus in which successive frames and possibly dropped frames may be more reliably detected by the apparatus. The method should also enable the use of very simple test setup.
In this context the term frame is one set of visual information in matrix form to be shown by a display. The frame may be a part of a video content i.e. one image of a sequence of images, a picture, etc. The term dropped frame or an omitted frame means a frame of a sequence of frames which is not displayed by the display although the frame is provided to the display. The reason for dropping the frame may be, for example, that the display is not fast enough, an input buffer of the display is full, the device controlling the display is occupied by other simultaneous tasks etc.